In all recommended guidelines put forth for the treatment of cancer pain, opioids continue to be an important part of a physician's armamentarium. Though opioids are used regularly for cancer pain, there is a paucity of literature proving efficacy for long-term use. Cancer is no longer considered a "terminal disease"; 50% to 65% of patients survive for at least 2 years, and there are about 12 million cancer survivors in the United States. There is a concern about side effects, tolerance, abuse and addiction with long-term opioid use and a need to evaluate the effectiveness of opioids for cancer pain. The objective of this systematic review was to look at the effectiveness of opioids for cancer pain. A systematic review of randomized trials of opioids for cancer pain. A comprehensive review of the current literature for randomized controlled trials (RCTs) of opioids for cancer pain was done. The literature search was done using PubMed, EMBASE, Cochrane library, clinical trials, national clearing house, Web of Science, previous narrative systematic reviews, and cross references. The studies were assessed using the modified Cochrane and Jadad criteria. Analysis of evidence was done utilizing the modified quality of evidence developed by United States Preventive Services Task Force (USPSTF). Pain relief was the primary outcome measure. Secondary outcome measures are quality of life (QoL) and side effects including tolerance and addiction. The level of evidence for pain relief based on the USPSTF criteria was fair for transdermal fentanyl and poor for morphine, tramadol, oxycodone, methadone, and codeine. Randomized trials in a cancer setting are difficult to perform and justify. There is a paucity of long-term trials and this review included a follow-up period of only 4 weeks. This systematic review of RCTs of opioids for cancer pain showed fair evidence for the efficacy of transdermal fentanyl and poor evidence for morphine, tramadol, oxycodone, methadone, and codeine.